Retractable strake and method

ABSTRACT

A retractable strake is disclosed that includes an assembly of flexible finger elements or flexible faces. The assembly, when coupled to a structure, deflect at wind speeds greater than approximately twenty mph or wind pressures greater than approximately one psf. Accordingly, the strakes&#39; retractable or flexing feature does not contribute to the wind load when the wind load is overly severe, while providing vortex shedding benefits within a wind speed profile range.

[0001] This non-provisional Continuation-in-Part application claims thebenefit and priority of pending U.S. patent application Ser. No.10/283,202 filed Oct. 30, 2002, titled, “RETRACTABLE RADOME STRAKE ANDMETHOD,” by Jeffrey H. Steinkamp and James C. Butts, the contents ofwhich are incorporated by reference in its entirety.

FIELD OF THE FIELD OF THE INVENTION

[0002] The present invention relates generally to methods and devicesfor reducing vortex shedding. More particularly, the present inventionis directed to a retractable strake for reducing the susceptibility ofan object to vortex shedding.

BACKGROUND OF THE INVENTION

[0003] It is well known that antenna structures or other tallcantilevered cylindrical structures are subject to vibration causedvortex shedding from wind forces. Vortex shedding refers to thephenomenon that occurs when wind forces exert an unbalanced crosswindpressure on opposite sides of the object. Vortex shedding occurs as theair moving over the surface of a cylinder separates from its hindquarters at about the four to five and seven to eight o'clock positionsforming a regular train of downwind swirling eddies or vortices. Theseparation of the vortices for a given size cylinder is periodic over arange of windspeeds. The pressure exerted by the air is reduced as itseparates from the cylinder. Thus the separating vortices causeunbalanced pressure fluctuations on the sides of the cylinder. If thefrequency of the pressure fluctuations is near or at the naturalfrequency of the structure, large amplitude, potentially damaging crosswind vibrations can occur.

[0004] For example, an antenna structure is typically surrounded by aradome. A radome is a hollow cylindrical mast, typically made fromfiberglass, that is placed around an antenna structure to protect itfrom elements, such as snow and ice, that could affect the performanceof the antenna. When a radome enclosed antenna structure is erected andsubjected to wind, the wind flows around the circumference of theradome.

[0005] As shown in FIG. 1, when wind 10 flows around the radome itseparates on the downwind surface. As the air separates from thecylinder a vortex is created. This typically occurs on one side at atime. As the newly created vortex moves away downwind another vortex isforming as the air separates from the opposite side of the cylinder.Each time this occurs the side to side pressure on the cylinder isunbalanced. FIG. 1 shows a possible pattern of vortices created. Thefrequency of the shedding vortices is dependent on the kinematicviscosity of the fluid (in this case air), the wind speed, and thegeometry of the object. The frequency of vortex shedding can be eitherrandom or periodic. For a fixed geometry (ie radome or smoke stack) thefrequency is most strongly dependent upon windspeed.

[0006] Antenna structures are designed to withstand established maximumexpected wind speeds as the local and national standards dictate. Theantenna structures are designed to withstand the expected maximum windspeeds, which are measured from a reference point location at or nearground level, occurring over a given time period of fifty years or so.Typically, the maximum design wind speeds are in excess of seventy mph.The wind pressure at the antenna is scaled up to account for theincrease in wind speed that occurs as with the increased height of thestructure.

[0007] Vortex shedding frequencies are either random or significantlyhigher than any of the potentially damaging modes of structuralvibration at the expected maximum windspeed used for structural design.The greatest vortex shedding problem occurs at low wind speeds. Thefrequency of vortex shedding is periodic at low wind speeds. A vortexwill shed off of one side and then the other at regular intervals,producing a periodic oscillating side to side force. This can bedamaging if the frequency of vortex shedding is slightly above the firststructural mode and the wind speed driving the structure is greater thanten mph. This will cause resonance, a condition where there is verylittle resistance to oscillatory motion. Large displacements can developcausing damage or failure. As shown in FIG. 1, the vortices 14, 16, 18are spiraling circles of wind that tend to increase the pressure exertedon the radome 12.

[0008] When the pressure on one side of a structure differs from thepressure on the opposite side of the structure, at a point in time, thestructure may move in a direction toward the side that is lower inpressure. As the wind traverses the structure, the pressure exerted onopposite sides of the structure may continue to fluctuate, and cause thestructure to vibrate, i.e., sway in response to the alternating lowpressure sides. For example, as shown in FIG. 1, vortex 14 will causeradome 12 to move downward, while vortex 16 will cause the radome 12 tomove upward.

[0009] Several approaches have been used to eliminate or alleviatevortex shedding. One of the most common is to use Scruton Windings,which are helical rigid strakes added to the upper third of thestructure to disrupt the formation of the vortices. The rigid strakesdisrupt and diffuse the flow of wind around the radome, such that thedevelopment of periodic vortices, which may cause the antenna structureto resonate, is reduced. The negative side of using helical rigidstrakes is a large amount of windload is added since the overallprojected area has increased as well as the shape is changed from alower drag full cylinder to more flattened cross-section.

[0010] Typically, radome enclosed antenna structures, such as atelevision broadcasting antennas, experience vortex shedding at windspeeds in the range of ten to twenty mph and/or at wind pressures at orbelow one pound per square foot (psf). Thus, strakes are mostly neededat wind speeds below approximately twenty mph and/or wind pressuresbelow one psf.

[0011] However, since the addition of the strakes to a radome increasesthe cross-sectional area of the radome, the radome is susceptible togreater wind loads that could affect the stability of the antenna. Thus,components of the antenna structure, such as an antenna mast and asupporting tower structure, have to be built stronger to withstand theincreased wind loads. As a result of the added strakes, the cost tomanufacture the antenna structure, or any structure that endures windforces, increases.

[0012] Accordingly, it would be desirable to provide a strake that mayreduce the susceptibility of wind-bearing structures such as towers,chimneys, smoke stacks, masts, etc. to vortex shedding, while reducingthe contribution of the strake to the wind load of the structure.

[0013] Further, it would be desirable to provide a strake that helps toprevent vortex shedding without significantly increasing the costs ofassociated structures, such as in the case of an antenna, the antennamasts and supporting tower structures.

SUMMARY OF THE INVENTION

[0014] In various embodiments of the present invention, system(s) forreducing vortex shedding on an object are provided. In one aspect of thepresent invention, the system comprises a strake having a substantiallypolygonal shape, and flexible attachment feet, wherein the strake isattached to the feet and the feet are attached to the object.

[0015] In another aspect of the present invention, the system comprisesvortex shedding means for shedding vortices about the object, whereinthe shedding means is flexibly retractable, and attachment means forattaching the vortex shedding means to the object, wherein at least oneof the vortex shedding means or the attachment means flexs when at leastone of wind speeds are greater than approximately twenty mph and windpressures are greater than approximately one psf.

[0016] In another aspect of the present invention, a method is providedfor manufacturing the present invention, comprising a method formanufacturing an apparatus for reducing vortex shedding on an object,comprising arranging a plurality of truncated two-side strake elementsinto an assembly of attachment elements, and coupling the assembly ofattachment elements to the object, such that the assembly of strakeelements deflects when at least one of wind speeds are greater thanapproximately twenty mph and wind pressures are greater than one psf.

[0017] There has thus been outlined, rather broadly, the more importantfeatures of the invention in order that the detailed description thereofthat follows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described below andwhich will form the subject matter of the claims appended hereto.

[0018] In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of otherembodiments and of being practiced and carried out in various ways.Also, it is to be understood that the phraseology and terminologyemployed herein, as well as the abstract, are for the purpose ofdescription and should not be regarded as limiting.

[0019] As such, those skilled in the art will appreciate that theconception upon which this disclosure is based may readily be utilizedas a basis for the designing of other structures, methods and systemsfor carrying out the several purposes of the present invention. It isimportant, therefore, that the claims be regarded as including suchequivalent constructions insofar as they do not depart from the spiritand scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 illustrates a radome subjected to vortex shedding.

[0021]FIG. 2 is a top view of a retractable strake in accordance withthe present invention.

[0022]FIG. 3 is a front elevation view of a retractable strake inaccordance with the present invention.

[0023]FIG. 4 is a front view and side views of another retractablestrake in accordance with the present invention.

[0024]FIG. 5 is a series of perspective views of the retractable strakeof FIG. 4 arranged about an antenna radome and a smoke stack, inaccordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0025] Referring now to the figures, wherein like reference numeralsindicate like elements, there is shown in FIG. 2, a retractable strake20, 22 in accordance with the present invention, that may be utilized toreduce vortex shedding. For purposes of example, the present inventionis described with respect to a radome 12. However it should beunderstood by one of ordinary skill in the art that a strake 20, 22 inaccordance with the present invention may have other applications, fortowers, chimneys, smoke stacks, masts, etc.

[0026] In an exemplary embodiment of the present invention, the strake20, 22 is constructed from an assembly of finger elements 24, 26, 28.The individual finger elements 24, 26, 28 are bristle elementsmanufactured from a non-metallic material, for example, a plastic, anylon material, or a polyethylene material. In another exemplaryembodiment of the present invention, the finger elements 24, 26, 28 areformed from strips of a plastic material, for example polyethylene.

[0027] It should be understood by one of ordinary skill in the art thata strake 20, 22, when utilized in connection with an antenna system, ismade from a non-metallic material to prevent interference with thetransmission of signals from the antenna. However, a strake 20, 22 ofthe present invention, when utilized for other applications, such aspreventing the occurrence of vortex shedding on, for example, metalchimney stacks, may be manufactured alternately from a metallicmaterial.

[0028] Shown in FIG. 2, the assembly of finger elements 24, 26, 28 arearranged according to a predetermined pattern. The pattern is designedsuch that the maximum height of the assembly of finger elements 24, 26,28 is approximately ten percent of the overall diameter of the radome12. In an exemplary embodiment of the present invention, the diameter ofthe radome 12 is forty inches and the maximum height of the assembly offinger elements is approximately four inches.

[0029] In an exemplary embodiment of the present invention, at least oneside of the assembly of finger elements 24, 26, 28 is curved, such thatthe strake 20, 22 can be curvedly positioned about the radome 12.

[0030] Shown in FIG. 3, strakes 20, 22, in accordance with presentinvention, are positioned on an exterior surface of a radome 12. In anexemplary embodiment of the present invention, the strakes 20, 22 arepositioned about the exterior surface of the radome, such that they forma helical or nearly helical pattern about the exterior surface of theradome.

[0031] By positioning the strakes 20, 22 in a helical-like type ofpattern about the radome 22, instead of straight out from the radome 12,the strakes 20, 22 cover more surface area of the radome 12, and areable to diffuse the wind flow, and prevent the development of vortices,such as vortices 14, 16, 18 shown in FIG. 1. Also, by configuring thestrakes in a semi-helical manner, the strakes are able to disrupt theair flow from any direction.

[0032] During operation, a strake 20, 22, in accordance with the presentinvention, is retractable. For example, at wind speeds of approximatelytwenty mph or less and/or wind pressures of approximately one pound psfor less, when vortex shedding typically occurs, the strake 20, 22 iserect, stiff and/or stable. Accordingly, the strake 20, 22 creates thenecessary turbulence to avoid the development of vortices that couldaffect the stability of, for example, a radome enclosed antennastructure.

[0033] However, the strake 20, 22 is designed such that, at wind speedsabove approximately twenty mph and/or wind pressures greater thanapproximately one psf, when vortex shedding typically does not occur,the strake 20, 22 deflects in the direction of airflow, as the windspeeds and/or wind pressures increase. Thus, the cross-sectional area ofthe radome 12, with the added strake, decreases. Accordingly, the amountof wind load that the radome 12 is susceptible to also decreases. Thedeflection serves to retract the strake.

[0034] In an exemplary embodiment of the present invention, at windspeeds of approximately twenty miles per hour, and/or wind pressures oftwelve and one-half psf, the assembly finger elements 24, 26, 28 of astrake 20, 22 completely deflect, and lay along the surface of theradome 12.

[0035] In an exemplary embodiment of the present invention, a strake20,22 is coupled to the radome via an adhesive. In an exemplaryembodiment of the present invention the radome 12 has openings/portsthrough which the finger elements 24, 26, 28 are inserted, and securedwith adhesive, such as an epoxy. In a second exemplary embodiment of thepresent invention, the individual finger elements 24, 26, 28, of astrake 20, 22, are secured to the exterior surface of the radome 12 withan adhesive.

[0036]FIG. 4 is a front view and side views of an exemplary retractablestrake 41. The retractable strake 41 is illustrated in the front view asbeing upright and having a generally rectangular shape. The retractablestrake 41 is a preferably, but not necessarily, made from a UV resistantelastomer and is fabricated in relatively short sections. In thisexample, the retractable strake 44 is of approximately 5 inches inlength or less. The retractable strake 41 employs a notch running alongthe edge adjacent to the surface of the structure 43 to help elevate thestrake 41, to add flexibility to the strake 41, and also to provide analternate flow path around the strake 41, and is attached to the surfacevia mounts 45. The mounts 45, shown here for illustrative purposes, asbolts or hex head screws secure the strake 41 and joined to the strake41 through the feet 47, which are attached to the strake 41 at itsperimeter. The feet 47 are made from a material that is pressuresensitive, specifically wind loads, to result in the strake 41 having abent configuration as shown in the high speed illustration. The feet 47serve as both the hinge point and spring tension to return the strake 41to its original low vortex shedding position.

[0037]FIG. 5 is a series of perspective views of an exemplary embodimentof this invention arrayed about an antenna radome 50 and a smoke stack60, respectively. The individual strakes 41 are placed around theantenna radome 50 and smoke stack 60 in an overlapping helical patternto provide vortex shedding from any angle of wind impingement. Thestrakes 41 are positioned with a slight gap between adjacent strakes 41to enable the adjacent sections to fold without interference. The gap inthe exemplary embodiments is about 0.50 inches, however, the gap may bechanged according to design preferences. Similar to the benefitsafforded to the antenna radome 50, the smoke stack 60 can be protectedfrom oscillatory vibrations caused by vortex shedding.

[0038] While FIG. 5 shows the exemplary retractable strakes according tothis invention being placed about an antenna radome 50 and a smoke stack60, it should be appreciated that the exemplary retractable strakes maybe placed about other structures, such as sailing masts, towers,buildings, etc., to provide vortex shedding. Also, while the exemplaryretractable strakes are shown in FIGS. 4 and 5 as having a predominantlyrectangular shape, alternate shapes may be used as deemed appropriate.Additionally, alternate positioning of the exemplary feet on the strakemay be facilitated based on the choice of materials used or the type ofapplication. For example, instead of using two feet, one foot may beused either at a midpoint of the strake edge or at another desiredlocation. Furthermore, it should be appreciated that while the exemplaryembodiments illustrated the use of flexible feet to accomplish aretractable function of the invention, other mechanisms may be usedwithout departing from the spirit and scope of this invention. Forexample, mechanical springs may be attached to the strake to enable thestrake to retract under a given wind load. Alternatively, portions of asection of the face of the strake may be made of a material that allowsthe strake to flex when subject to a given wind load rather than theentire face of the strake.

[0039] Furthermore, while the surface of the strake in FIGS. 4-5 isshown as having a smooth face, alternative strake face designs may beused utilizing any one of holes, grooves, dimples, patterns, etc. thatresult in a distinct wind drag attribute to the strake. Thus, strakesaligned about a structure may have an assortment of wind effectprofiles. For example, alternate strakes in the helical pattern may flexat different wind speeds, according to design preference. Additionally,it should be appreciated that a non-helical arrangement of strakes maybe implemented, or a non-continuous arrangement, as desired.

[0040] In various exemplary embodiments of the present invention, astrake may be assembled on a non-metallic support structure and/or basethat is molded into the structure of, or coupled to the wind-bearingobject with non-metallic hardware. In various other exemplaryembodiments of the present invention, the strake is assembled within anon-metallic frame structure that is coupled to the wind-bearing objectwith non-metallic hardware. It should be understood by one of ordinaryskill in the art that there may be various other methods for couplingthe strake to a wind-bearing object.

[0041] The many features and advantages of the invention are apparentfrom the detailed specification, and thus, it is intended by theappended claims to cover all such features and advantages of theinvention which fall within the true spirit and scope of the invention.Further, since numerous modifications and variations will readily occurto those skilled in the art, it is not desired to limit the invention tothe exact construction and operation illustrated and described, andaccordingly, all suitable modifications and equivalents may be resortedto, falling within the scope of the invention.

What is claimed is:
 1. A system for reducing vortex shedding on anobject, comprising: a strake having a substantially polygonal shape; andflexible attachment feet, wherein the strake is attached to the feet andthe feet are attached to the object.
 2. The system of claim 1, whereinthe strake deflects at wind pressures greater than approximately onepsf.
 3. The system of claim 1, wherein the strake deflects at windspeeds greater than approximately twenty mph.
 4. The system of claim 1,wherein the strake's inner edge is notched.
 5. The system of claim 1,wherein the strake is attached to the object by a single foot.
 6. Thesystem of claim 1, wherein the strake is substantially rectangular. 7.The system of claim 1, further comprising: a plurality of strakes, theplurality of strakes being arranged about the object in a semi-helicalform.
 8. The system of claim 1, wherein the plurality of strakes arearranged about the object in a plurality of semi-helical forms.
 9. Thesystem of claim 1, wherein the strake is molded to the feet.
 10. Thesystem of claim 1, wherein the feet are molded into the object.
 11. Thesystem of claim 1, wherein the strake is less than five inches wide. 12.The system of claim 1, wherein a maximum height of the strake isapproximately 10 percent of an overall diameter of the object.
 13. Thesystem of claim 1, wherein a surface of the strake is patterned.
 14. Thesystem of claim 1, wherein the object is a radome.
 15. The system ofclaim 1, wherein the object is a chimney.
 16. The system of claim 1,wherein the object is a mast.
 17. The system of claim 7, wherein thestrakes of the plurality of strakes are displaced from each other byapproximately {fraction (1/2)} inches.
 18. An apparatus for reducingvortex shedding on an object, comprising: vortex shedding means forshedding vortices about the object, wherein the shedding means isflexibly retractable; and attachment means for attaching the vortexshedding means to the object, wherein at least one of the vortexshedding means or the attachment means flexs when at least one of windspeeds are greater than approximately twenty mph and wind pressures aregreater than approximately one psf.
 19. The apparatus of claim 17,wherein the shedding means and the attachment means are positioned in asemi-helical arrangement about the surface of the object.
 20. Theapparatus of claim 18, wherein the attachment means is an adhesive. 21.The apparatus of claim; 18, wherein the shedding means is primarily atruncated flat surface.
 22. The apparatus of claim 18, wherein theobject is a radome.
 23. A method for manufacturing an apparatus forreducing vortex shedding on an object, comprising: arranging a pluralityof truncated two-side strake elements into an assembly of attachmentelements; and coupling the assembly of attachment elements to theobject, such that the assembly of strake elements deflects when at leastone of wind speeds are greater than approximately twenty mph and windpressures are greater than one psf.